Apparatus for electrolysis of molten oxides

ABSTRACT

The invention provides improved electrodes for electrolytic cells operating with molten salt electrolytes. Nonconsumable iridium-based anodes of the invention facilitate the release of gaseous oxygen from oxide-containing melts, for example in the electro-chemical production of liquid or gaseous reactive metals from oxides. Cathode substrates of the invention are constructed of a tungsten-based alloy and enable deposition of an overlying liquid-metal cathode. Incorporation of the anode and cathode substrate of the invention into molten-oxide cells establishes a novel method for electrolytic extraction of titanium and other reactive metals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrolytic cells for the production ofmetals from molten electrolytes. In particular, this invention providesnovel anodes and cathode substrates for use with oxide melts and liquidor gaseous metal products. More particularly, an electrochemicaltechnique for deposition of liquid titanium from titanium dioxide isdescribed.

2. Background Information

Electrolytic processes acting on molten salt electrolytes have been usedto produce several important metals, examples of which are given in U.S.Pat. No. 5,185,068, the entire disclosure of which is incorporated byreference.

Electrolytic reduction in extractive metallurgy has seen its greatestcommercial success in the production of aluminum. The invention of theHall-Héroult process in the 1880s gave birth to a new industry andtransformed aluminum from a precious metal into a ubiquitous material,commonplace in construction, transportation, packaging, and electricaldevices.

This process functions by passing an electric current through a moltenfluoride electrolyte containing alumina at about 1000° C. (Theelectrochemical extraction of aluminum by the Hall-Héroult process isdiscussed in U.S. Pat. No. 4,999,097, the entire disclosure of which isincorporated by reference.) The Hall-Héroult process has persisted asthe only commercial production method for aluminum with little change inits components since early implementations. For example, the anode atwhich oxygen from the melt is oxidized is made of carbonaceous materialsas were the first Hall-Héroult cell anodes. Carbon has several usefulproperties that have allowed the Hall-Héroult process to become atonnage provider of aluminum. Carbon anodes are solid at the celloperating temperatures, well above the melting point of aluminum, sothat the aluminum can be retrieved in liquid form. Also, the electricalconductivity of carbon is sufficiently high that the massive amperagesrequired for practicable production rate can be conveyed to the meltwithout excessive voltages.

However, significant problems also attend the use of carbon anodes,related to their preparation and to their consumption during aluminumsmelting. Carbon anodes typically are fabricated by baking carbonaceousfeedstocks such as coal tar, petroleum coke and pitch at hightemperatures for extended periods of time. Arranging these materials fortreatment exposes workers to injurious carbon dust. Then, during thisprebake process the anode outgases undesirable by-products such ashydrocarbons, polychlorinated biphenyls and sulfur dioxide.Consequently, anode fabricators must undertake expensive filtering,collecting and treatment operations. Furthermore, anode outgassinggenerally persists throughout the active lifetime of the electrode.

Moreover, interactions between the anode and the molten electrolyteduring cell operation consume the anode. Although, ideally, the anodereaction should be the oxidation of oxide ions to gaseous oxygen, thecarbon anode is not inert in the aggressive high-temperature chemicalenvironment of the cell. Anode material is removed from the electrode asoxygen in the bath combines with carbon to form volatile carbon monoxideand carbon dioxide. Similarly, perfluorocarbon compounds, mainlytetrafluoromethane and hexafluoroethane, are produced during operationat low alumina concentration, so that aluminum smelting is the greatestsingle contributor to perfluorocarbon compound emissions in the UnitedStates.

Anode consumption is problematic for several reasons. First, it makescell operation more difficult. It is difficult to maintain uniform anodecurrent loading during operation due to the continuously changingtopology of the electrolyte-anode interface. Due to uncertainty in theposition of the anode's inconstant surface, the anode-cathode spacing ismaintained at a cautiously large value to preclude electrical shortingbetween the anode and cathode so that a greater cell voltage must beapplied to the cell to drive current through the electrolyte. The anodemust be continually repositioned to maintain contact with the bath asits vertical dimension shrinks. Anode changes are labor intensive anddisturb the thermal balance and electrical current distribution in thecell. Gases produced by the consumption reaction can remain trappedunder the anode surface and contribute additional ohmic drops to theoperating voltage. With about one-half pound of solid carbon beingconverted to so-called greenhouse gases for every pound of aluminumproduced by the Hall-Héroult process, environmental concerns aboutelectrolytic aluminum smelting as historically practiced have becomemore prominent.

Due to these problems, considerable research has been directed towardfinding an alternate anode material. In addition to maintaining thedesirable characteristics of the currently used anode material, theideal electrode would be nonconsumable, serving mainly as an electronsink. Such an electrode would not react with oxygen formed at the anodeand would not dissolve in the molten electrolyte. Despite substantialeffort, no fully satisfactory inert anode has been identified.

Owing to the great advantages of the electrolytic process overalternative aluminum smelting schemes, the Hall-Héroult cell haspersisted despite the longfelt need for an alternative to carbon. Whilean inert anode has been viewed as a highly desirable target, itsdiscovery would provide an enhancement to an already-workable system—nota pre-requisite for a viable process. However, the identification ofsuch an oxide-compatible, high-temperature-stable electrode for aluminumextraction could also make possible new processes for extraction ofother metals from their oxide compounds.

The example of titanium is instructive. The main sources of titanium areilmenite and rutile, in both of which titanium is bound to oxygen.Nonelectrolytic industrial processes for titanium extraction act ontitanium chlorides and so require an additional preliminary unitoperation, the carbochlorination of titanium dioxide—which generatescarbon dioxide—for rendering titanium chloride feed for the finalreduction. For example, the principal commercial route by which titaniumis produced is the Kroll process, a batch metallothermic reduction oftitanium tetrachloride by magnesium. The titanium sponge product iscontaminated with excess magnesium and magnesium chloride.

Of the electrolytic titanium-smelting processes that have beenevaluated, most do not directly reduce a titanium-oxygen compound butrather begin with a chlorinated compound. For example, the Dow-Howmetprocess dissolves titanium dichloride, produced from the tetrachloride,in a potassium and lithium chloride molten electrolyte. With anoperating temperature of about 520° C., the Dow-Howmet cell, like otherelectrolytic titanium producing operations, deposits titanium well belowits melting temperature (1670° C.), resulting in dendritic soliddeposits into which electrolyte is entrained. The product must be washedand then remelted to render metal ingot.

The Fray, Farthing, & Chen (FFC) process differs in that it usestitanium dioxide feedstock in the form of a reactive cathode immersed ina molten chloride electrolyte containing calcium chloride. By passage ofcurrent, oxygen is electrochemically removed from the TiO₂ cathodeleaving behind elemental titanium. On the anode, oxygen reacts as itdoes in the Hall cell to produce CO₂. Although the FFC process moves inthe right direction by obviating chlorination of titanium dioxide it isnonetheless environmentally suspect due to the halide electrolyte. Theformation of unacceptable, albeit small, amounts of dioxin and furansundoubtedly attends the evolution of carbon dioxide on the carbon anodein such an environment. Also, the FFC process has not becomeeconomically viable owing to the long times required to remove all theoxygen from the cathode, solid-state diffusion being extremely slow.

Identification of an inert anode stable in a molten oxide environment attemperatures in the liquid range of titanium would eliminate the needfor carbon and molten halides in the electrolytic extraction titanium.Using such an anode, liquid titanium could be obtained directly from anoxide source with gaseous oxygen as a byproduct. However, such hightemperatures and highly corrosive conditions constitute a set of designconstraints even more stringent than those for an inert anode for thecomparatively benign physicochemical climate of the Hall-Héroult cell.Even if the standard noble metals—gold, silver and platinum—were notconventionally regarded as prohibitively expensive for large-scaleindustrial applications, they would not be suitable candidates for usein a liquid titanium cell owing to either their relatively low meltingtemperatures (silver and gold) as compared with that of titanium or, inthe case of platinum, to its lack of structural integrity.

Beyond designating an inert anode, the design of apparatus forelectrolytic extraction from oxide media poses many challenges not facedin the Hall-Héroult cell. For example, another essential component withrigorous materials requirements is the cathode substrate upon which themetal is deposited, the deposit subsequently acting as the cathode.

An ideal cathode substrate would have the following qualities: first,the cathode substrate material must be solid at the operatingtemperature of the electrolytic cell. Considering titanium again as anillustrative example, materials that may be serviceable at aluminum'srelatively low melting temperature may well lack structural integrity atthe higher temperatures—exceeding 1700° C.—required for deposition oftitanium metal in liquid form.

Another requirement of the cathode substrate material is that it notreact chemically with the molten electrolyte or the liquid metalproduct. The carbon substrate of the Hall-Héroult cell is not compatiblewith reactive metals such as titanium in this instance because thesemetals react, to an undesirable extent, with carbon to form carbides.

A third requirement concerns the electronic conductivity of the cathodesubstrate material. The rate of electrolytic metal deposition isproportional to the flow of electrons through the cathode substrate. Ifthe electronic conductance of this element is too low, metal depositionat a commercially acceptable rate will be achieved only by applicationof a large voltage, which in turn will translate into unacceptably highelectric power cost. Thus, in order to maintain an economical energyefficiency for metal extraction, the substrate material must bereasonably conductive.

Materials from the class of compounds known as refractory hard metals(RHMs) have been considered as candidate materials for the cathodesubstrate in liquid metal electrodeposition apparatus based on thesecriteria. However, RHMs have other properties that detract from theirsuitability for cathode support in electrolytic cells: RHMs areexpensive and also mechanically brittle and therefore difficult toshape.

To date, no material has been shown to meet the performance andpractical requirements for a cathode substrate in a cell containing anoxide melt at temperatures exceeding 1700° C.

DESCRIPTION OF THE INVENTION OBJECTS OF THE INVENTION

An object of the present invention is, accordingly, an apparatus forextraction of metal from an oxide feedstock without use of carbon-basedelectrodes.

Another object of the invention is an electrochemical apparatus fordepositing liquid titanium from titanium dioxide feedstock.

Another object of the invention is the electrochemical extraction ofmetals, in liquid or gaseous form, above their melting temperatures.

Another object of the invention is metal smelting with significantlyreduced emission of greenhouse gases.

Another object of the invention is nonconsumable anodes forelectrochemical extraction of metals.

Still another object of the invention is an anode shaped with channelsfor ducting evolved oxygen.

Another object of the invention is a means for electrochemicalproduction of oxygen by the action of electric current through anelectrode immersed in a molten electrolyte containing a source ofoxygen.

Another object of the invention is to provide conductive cathodesubstrates compatible with liquid reactive metals and moltenreactive-metal precursors.

BRIEF SUMMARY OF THE INVENTION

The invention relates to the discovery that materials heretofore notconsidered useful for electrodes and electrode substrates of cells forthe electrolytic production of metals from oxide-based feedstocks in amolten electrolyte can serve as such, thereby providing improvedelectrolytic cells and novel methods for metal production.

In one aspect, the invention provides nonconsumable iridium anodes foraccepting electrons from oxide-containing melts, thereby facilitatingthe oxidation of precursors in the electrolyte, with evolution ofgaseous oxygen. The anodes of the invention comprise an iridium-basedbody having a continuous iridium surface, such that substantially theentire anodic surface in contact with the electrolyte in the cell is aniridium-based material.

As used herein, the term iridium-based denotes a material comprisingiridium sufficiently concentrated and voluminous to establish asubstantially continuous iridium surface over a contactinterface—defined to be the entire anode-electrolyte interface—and alsoto confer mechanical integrity and chemical properties of the same orderas those of pure bulk iridium. In one embodiment, the anode issubstantially pure iridium. In another embodiment, the anode comprisesan electrode foundation of a less-expensive material overlaid by acontinuous film of iridium-based material. In yet another embodiment,the anode is made of an iridium-based alloy having an iridium content ofat least 80%.

With a melting point of 2446° C., iridium metal can withstand servicetemperatures for electrolytic smelting of reactive metals withoutdegradation of its mechanical properties. Its excellent high-temperatureoxidation resistance allows it to serve in the aggressive molten oxideenvironment required to achieve the objects of the invention.

The electrodes disclosed herein eliminate the need for the expensivededicated anode-preparation shop necessary to maintain a smeltingfacility using carbon anodes. Being essentially nonconsumable, theanodes are replaced less frequently, and operating costs are reduced.The stable anode contour permits closer approach to the metal product sothe cell can function at lower voltage and hence lower power cost perunit of metal product.

The mechanical properties of iridium make possible a variety of anodegeometries greatly advantageous over the block design used for carbonanodes. In one embodiment, the anode is formed with channels forconveying oxygen evolving at the anode-electrolyte interface out of thecell, preventing the aggregation of large bubbles that reduce cellefficiency. A similar advantage is provided by another embodiment inwhich the anode has a comb-like structure.

In one embodiment, the inert anode of the invention is incorporated intoa conventional Hall-Héroult cell for aluminum extraction and replacescarbon-based anodes of the prior art, with improved performance withrespect to difficulty of operation and undesirable emissions.

In another embodiment, the iridium-based anode contacts a moltenelectrolyte containing titanium and oxygen precursors as part of a cellproducing titanium metal, preferably operating at a temperaturesufficient for production of liquid titanium. In a preferred embodiment,the anode is used with an electrolyte comprising titaniumdioxide—derived, for example, from anatase or rutile—dissolved in amolten oxide solvent containing at least one member of the groupconsisting of beryllium oxide, magnesium oxide, calcium oxide, aluminumoxide, and lithium oxide.

In yet another embodiment, the iridium-based electrode of the inventionserves as anode in an apparatus evolving gaseous oxygen, for example inconjunction with deposition of a reactive metal. Candidate reactivemetals for liquid-phase deposition include beryllium, aluminum, silicon,scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel,copper, gallium, germanium, yttrium, zirconium, hafnium, indium, tin,barium, lanthanum, cerium, praseodymium, neodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,lutetium, actinium, thorium, protactinium and uranium. The anode of theinvention could similarly serve in an electrolysis apparatus forvapor-phase production of reactive metals such as lithium, sodium,magnesium, potassium, calcium, rubidium, strontium and cesium.

In this embodiment, the anode of the invention is equally suitable forcell configurations incorporating one or more vertically oriented,dipping cathodes, which furnish electrons for the electrochemicalreduction of ions in the electrolyte but do not act as a substratesupporting the accumulated metal deposit. Such a configuration may bemore satisfactory in cells operating so as to produce a vapor-phasemetal since the proximity of the cathode to the upper surface of theelectrolyte simplifies collection of the metal product, which canreadily exit the cell at the top. Dipping electrodes may be similarlydesirable in the case of a liquid metal product lower in density thanthe molten oxide electrolyte. Cells may additionally include aprotective wall between anode and cathode for shielding products of therespective electrodes from one another to prevent recombination orelectrical shorting. Such design considerations are well known to thoseskilled in the art of electrochemical extraction of metals.

In another aspect, the invention provides tungsten-based cathodesubstrates—having a tungsten content of at least 50%—for deposition ofliquid reactive metals. The cathode substrate furnishes electrons to themolten electrolyte, either directly or through a liquid metal layer,during cell operation. Cathode substrates constructed of atungsten-based alloy enable deposition of an overlying liquidreactive-metal cathode without continual formation of carbides to anextent that would erode the cathode substrate. Such materials areresistant to alloying with liquid titanium and are solid at thetemperature range of interest. Furthermore, unlike elemental tungsten,they resist oxidation at high temperatures. In a preferred embodiment,the tungsten-based alloy contains at least 15% rhenium by weight; anexample is W-25Re, comprising 75% tungsten and 25% rhenium, which has amelting temperature above 3000° C.

Incorporation of the anode and cathode substrate of the invention intoan electrolysis cell containing molten-oxide electrolyte establishes anovel method for electrolytic extraction of reactive metals. In apreferred embodiment, the iridium-based anode and tungsten-based cathodesubstrate co-operate in a cell for optimum production of liquid titaniummetal directly from an oxide feedstock. Titanium production according tothe invention proceeds without release of greenhouse gases: Because themetal is reduced directly from oxide, preparatory processing is muchcleaner and simpler. Since the invention deposits titanium in liquidform, subsequent treatment is far easier than in current methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing discussion will be understood more readily from thefollowing detailed description of the invention, when taken inconjunction with the accompanying drawings, in which like referencenumerals indicate identical or functionally similar elements, of which:

FIG. 1 is a vertical section showing a Hall-Héroult cell such as isemployed commercially for aluminum extraction;

FIG. 2 is a vertical section illustrating one embodiment of aHall-Héroult cell modified to employ a nonconsumable anode of thisinvention, the single anode having venting channels;

FIG. 3 is a vertical section illustrating one embodiment of a Sadowaycell for titanium extraction according to the invention; and

FIG. 4 is a vertical section illustrating the anode of the inventionused in a cell having a dipping cathode.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a conventional Hall-Héroult cell 10, of theprior art, has a steel outer shell 12 lined by overlying thermalinsulation 14. A carbon cathode substrate 16 positioned at the bottom ofthe cell 10 contains metallic current collector bars 18. Carbon anodes20 are formed by prebaking carbon blocks suspended from steel anode rods22 which supply electrical current to the anodes 20. A cell lining 24 isalso formed from carbon blocks.

Molten electrolyte 26 contains dissolved alumina, which is continuallysupplied by breaking an alumina crust 28 and adding fresh alumina. Thealumina crust 28 forms on frozen electrolyte and helps to minimize heatloss from the top of the cell 10. Since cryolite, Na₃AlF₆, has thecapacity to dissolve alumina, it is the principal constituent of theelectrolyte 26. Additionally, certain fluoride salts are present in theelectrolyte 26. Calcium fluoride, CaF₂, decreases the freezing point ofcryolite. Aluminum fluoride, AlF₃ contributes to the freezing pointdepression and also improves current efficiency in the cell 10.

The operating temperature of the cell is about 1000° C. As electriccurrent passes from the carbon anodes 20 through the molten electrolyte26 to the cathode 16, dissolved alumina is reduced to form a moltenaluminum layer 32 over the cathode substrate 16 at the bottom of thecell 10. Gas consisting principally of carbon dioxide and carbonmonoxide is generated at the anode 20. The carbon anode 20 is consumedduring this reaction.

Contact between the molten electrolyte 26 and the carbon cell lining 24would result in a chemical attack on the cell lining 24 due to theformation of intercalation compounds. In order to prevent such contactthe cell 10 is operated under conditions that cause a frozen electrolytelayer 30 to form between the carbon cell lining 24 and the moltenelectrolyte 26. The molten electrolyte 26 is surrounded by the shell offrozen electrolyte 30 and supported by the molten aluminum layer 32. Theneed to maintain the frozen electrolyte layer 30 complicates control ofthe cell because the location of the interface between the molten andfrozen electrolyte phases varies with operating conditions of the cell10.

In practice, the carbon cathode substrate 16 is covered with a deep poolof molten aluminum so that aluminum deposits onto the molten aluminumlayer 32 rather than onto carbon. Thus, the molten aluminum layer 32provides electrons to the molten electrolyte and functions as cathode inthe cell 10. This arrangement prevents the degradation of the cathodesubstrate 16 by creation and dissolution into the electrolyte ofaluminum carbide, which is formed when aluminum is electrodeposited ontocarbon.

With reference to FIG. 2, in a preferred embodiment of the invention, anelectrolysis cell 40 configured for the extraction of aluminum has asteel outer shell 42 lined by overlying thermal insulation 44. A carboncathode substrate 46 positioned at the bottom of the cell 40 containsmetallic current collector bars 48. A single anode 50 is constructedfrom iridium metal and formed with channels 52 for venting oxygen to theextetior of the cell 40. The anode 50 is connected to a supply ofelectric current by an anode rod 54, which may be of iridium or of someother conductive material. Cell lining 56 is also formed from carbonblocks.

The cell contains molten electrolyte 58 covered with a frozen aluminalayer 60. Electrolyte compositions capable of dissolving alumina, suchas are found in conventional Hall-Héroult cells, are compatible with thecell of the invention. Accordingly, the cell 40 is operated so as tomaintain a frozen electrolyte layer 62.

The operating temperature of the cell is about 1000° C. As electriccurrent passes from the iridium anode 50 through the molten electrolyte58 to the cathode substrate 46, dissolved alumina is reduced to build amolten aluminum layer 64 over the cathode substrate 46 at the bottom ofthe cell 40. Gas consisting principally of oxygen is generated at theinert, nonconsumable anode 50 and passes through the channels 52, thusexiting the cell 40.

With reference to FIG. 3, in another preferred embodiment of theinvention, an electrolysis cell 70, configured for the extraction oftitanium has a tungsten-rhenium outer shell 72 lined by overlyingthermal insulation 74. A tungsten-rhenium cell interior, comprisingsidewalls 75 and a cathode substrate 76, nests inside the thermalinsulation 74. The cathode substrate 76 positioned at the bottom of thecell 70 contains metallic current collector bars 78. A single anode 80is constructed from iridium metal and formed with channels 82 forventing oxygen to the exterior of the cell 70. The anode 80 is connectedto a supply of electric current by an anode rod 84, which is of someelectrically conductive material.

The cell contains molten electrolyte 88 acting as a solvent for titaniumdioxide. The electrolyte 88 is preferably one or more molten oxides, theprecise composition of which is selected for its capability ofdissolving titanium dioxide as well as other physical and chemicalproperties known to those skilled in the art of molten saltelectrochemistry.

The operating temperature of the cell is about 1700° C. As electriccurrent passes from the iridium anode 80 through the molten electrolyte88 to the cathode substrate 76, dissolved titanium dioxide is reduced tobuild a liquid titanium layer 94 over the cathode substrate 76 at thebottom of the cell 70. Gas consisting principally of oxygen is generatedat the inert, nonconsumable anode 80 and passes through the channels 82,thus exiting the cell 70.

With reference to FIG. 4, in yet another embodiment of the invention, anelectrolysis cell 90 has an iridium anode 92 and a cathode 94 which aresuspended above the cell 90 and dip into a molten electrolyte 96. Duringcell operation, gaseous oxygen evolves on the anode 92. The cathode 94provides electrons to the electrolyte 96, thereby extracting a reactivemetal, which, depending on its boiling temperature, leaves the cell 90as a vapor or remains in the cell in the liquid phase. A protective wall98 interposed between the anode 92 and the cathode 94 shields productsof the respective electrodes from one another to prevent recombinationor electrical shorting.

Although specific features of the invention are included in someembodiments and drawings and not in others, it should be noted that eachfeature may be combined with any or all of the other features inaccordance with the invention.

It will therefore be seen that the foregoing represents a highlyadvantageous approach to providing anodes and cathodes forhigh-temperature molten-oxide metal extraction. The terms andexpressions employed herein are used as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding any equivalents of the features shown anddescribed or portions thereof, but it is recognized that variousmodifications are possible within the scope of the invention claimed.

1. An apparatus for extracting liquid titanium metal from titaniumdioxide by electrolysis, the apparatus comprising: a. a cathodesubstrate of a tungsten-based material, the liquid titanium metalforming a deposit over the cathode substrate; b. an anode ofiridium-based material upon which gaseous oxygen evolves; c. a moltenelectrolyte containing the titanium dioxide, the electrolyte being incontact with the anode over a contact interface, the contact interfacebeing a substantially continuous iridium surface, the titanium dioxidebeing dissolved in the molten electrolyte.
 2. The apparatus of claim 1wherein the tungsten-based material contains rhenium.
 3. The apparatusof claim 1 wherein the tungsten-based material is W-25Re.
 4. Theapparatus of claim 1 wherein the molten electrolyte comprises a moltenoxide solvent.
 5. The apparatus of claim 4 wherein the molten oxidesolvent comprises at least one member of the group comprising berylliumoxide, magnesium oxide, calcium oxide, aluminum oxide, and lithiumoxide.
 6. The apparatus of claim 1 wherein the iridium-based materialcontains at least 80% iridium.
 7. The apparatus of claim 1 wherein theapparatus has an exterior, the anode being constructed with channels forconveying the oxygen from the substantially continuous iridium surfaceto the exterior.
 8. The apparatus of claim 1 wherein the iridium-basedmaterial is substantially pure iridium.
 9. An apparatus for extracting areactive metal from an oxide feedstock by the action of electriccurrent, the apparatus comprising: a. a cathode substrate of atungsten-based material, the reactive metal forming a deposit over thecathode substrate; b. a molten electrolyte in which the feedstock isdissolved.
 10. The apparatus of claim 9 wherein the reactive metal istitanium.
 11. The apparatus of claim 10 wherein the deposit is liquidtitanium.
 12. The apparatus of claim 9 wherein the reactive metal is amember of the group comprising beryllium, aluminum, silicon, scandium,vanadium, chromium, manganese, iron, cobalt, nickel, copper, gallium,germanium, yttrium, zirconium, hafnium, indium, tin, barium, lanthanum,cerium, praseodymium, neodymium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium,actinium, thorium, protactinium and uranium.
 13. The apparatus of claim9 wherein the tungsten-based material contains rhenium.
 14. Theapparatus of claim 9 wherein the tungsten-based material is W-25Re. 15.An apparatus for producing oxygen by the action of electric current, theapparatus comprising: a. a molten electrolyte containing a source of theoxygen; b. an anode of an iridium-based material contacting theelectrolyte over a contact interface, the contact interface being asubstantially continuous iridium surface, the oxygen evolving on theanode.
 16. The apparatus of claim 15 wherein the electrolyte contains anoxide source of a reactive metal, the apparatus further comprising acathode substrate, a liquid deposit of the reactive metal depositforming over the cathode substrate by the action of the electriccurrent.
 17. The apparatus of claim 16 wherein the reactive metal isaluminum.
 18. The apparatus of claim 16 wherein the reactive metal istitanium.
 19. The apparatus of claim 16 wherein the reactive metal is amember of the group comprising beryllium, aluminum, silicon, scandium,titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper,gallium, germanium, yttrium, zirconium, hafnium, indium, tin, barium,lanthanum, cerium, praseodymium, neodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,lutetium, actinium, thorium, protactinium and uranium.
 20. An apparatusfor extracting a reactive metal from an oxide feedstock by the action ofelectric current, the apparatus comprising: a. a molten electrolyte inwhich the oxide feedstock is dissolved; b. a cathode contacting theelectrolyte; c. an anode of an iridium-based material contacting theelectrolyte over a contact interface, the contact interface being asubstantially continuous iridium surface, the oxygen evolving on theanode.
 21. The apparatus of claim 20 wherein the reactive metal isextracted as a vapor.
 22. The apparatus of claim 20 wherein the reactivemetal is extracted as a liquid.
 23. An apparatus for extracting liquidaluminum from an aluminum oxide feedstock by electrolysis, the apparatuscomprising: a. a molten electrolyte in which the feedstock is dissolved;b. an anode of iridium-based material contacting the electroyte over acontact interface, the contact interface being a substantiallycontinuous iridium surface.
 24. The apparatus of claim 1 wherein thetitanium dioxide is derived from anatase.
 25. The apparatus of claim 1wherein the titanium dioxide is derived from rutile
 26. The apparatus ofclaim 21 wherein the reactive metal is a member of the group comprisinglithium, sodium, magnesium, potassium, calcium, rubidium, strontium andcesium.
 27. The apparatus of claim 22 wherein the reactive metal is amember of the group comprising beryllium, aluminum, titanium silicon,scandium, vanadium, chromium, manganese, iron, cobalt, nickel, copper,gallium, germanium, yttrium, zirconium, hafnium, indium, tin, barium,lanthanum, cerium, praseodymium, neodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,lutetium, actinium, thorium, protactinium and uranium.